Our prior studies of carbocyclic analogs of purine and pyrimidine nucleosides have uncovered significant anticancer and antiviral activity among compounds with this structure, in which the furanose ring of nucleosides is replaced by a cyclopentane ring. The observed activities by certain carbocyclic analogs, the importance by pyrimidine nucleosides among clinically active anticancer drugs, and the preeminence of nucleosides among clinically useful antiviral agents indicate a potentially important role for carbocyclic nucleoside analogs. For these reasons, this collaborative research program is comprised of synthesis, biological evaluations, and biochemical studies of new carbocyclic analogs of nucleosides. Efforts in these areas are devoted principally to new carbocyclic pyrimidine nucleoside analogs that are either congeners of the active carbocyclic analogs or are analogs of known, active pyrimidine nucleosides. The new analogs are being evaluated in vitro for anticancer activity and for antiviral activity. Analogs that show significant activity in vitro are to be further evaluated in vivo. Biochemical studies are designed to determine if active analogs are enzymatically activated, to identify the locus, or loci, of action of active analogs, and to determine if antiviral analogs selectively inhibit virus-induced enzymes.